Disk-based storage devices such as hard disk drives (HDDs) are used to provide non-volatile data storage in a wide variety of different types of data processing systems. A typical HDD comprises a spindle which holds one or more flat circular storage disks, also referred to as platters. Each storage disk comprises a substrate made from a non-magnetic material, such as aluminum or glass, which is coated with one or more thin layers of magnetic material. In operation, data is read from and written to tracks of the storage disk via a read/write head that is moved precisely across the disk surface by a positioning arm as the disk spins at high speed. The storage capacity of HDDs continues to increase, and HDDs that can store multiple terabytes (TB) of data are currently available.
HDDs often include a system-on-chip (SOC) to process data received from a computer or other processing device into a suitable form to be written to the storage disk, and to transform signal waveforms read back from the storage disk into data for delivery to the processing device. The SOC has extensive digital circuitry and has typically utilized advanced complementary metal-oxide-semiconductor (CMOS) technologies to meet cost and performance objectives. The HDD also generally includes a preamplifier that interfaces the SOC to the read/write head used to read data from and write data to the storage disk. As is well known, the read/write head may comprise, for example, separate read and write heads.
One control function of the HDD that is typically implemented in or otherwise supported by the preamplifier is electronic fly height control. As the operating temperature of the HDD changes, it is desirable to keep the fly height, or spacing between the read/write head and the storage disk surface, as constant as possible. In an exemplary fly height control arrangement, a resistive heating element is incorporated near a pole tip of the read/write head such that the space between the read/write head and the surface of the storage disk can be electronically controlled via thermal expansion.
A bandgap reference circuit is typically utilized to provide a voltage reference for the fly height control. A bandgap reference may be generated by combining two unlike quantities, such as a voltage that is proportional to absolute temperature (PTAT) and a voltage that is complementary to absolute temperature (CTAT). Similar results can be achieved using currents instead of voltages as the PTAT and CTAT quantities that are combined to provide the bandgap reference.
In either case, the PTAT and CTAT quantities combine to produce a reference that is substantially independent of temperature, at least to first order. However, the PTAT and CTAT quantities do not completely cancel one another. For example, while it is possible to produce a PTAT voltage that is highly linear with temperature, a CTAT voltage is more difficult to produce, and generally does not exhibit as high a degree of linearity with temperature as the PTAT voltage, particularly when implemented in a silicon integrated circuit. Such differences in linearity between the PTAT and CTAT voltages with temperature tend to produce a bandgap voltage reference having a temperature response curve that exhibits an undesirable downward bowing effect.
A conventional bandgap reference circuit may therefore not provide a sufficiently stable output over temperature, leading to difficulties in maintaining stable fly height in the HDD. Similar performance problems can arise in other types of control applications that utilize a bandgap reference circuit.